The development of the olfactory organs in newly hatched monotremes and neonate marsupials

Abstract

Olfactory cues are thought to play a crucial role in the detection of the milk source at birth in mammals. It has been shown that a marsupial, the tammar wallaby, can detect olfactory cues from its mother's pouch at birth. This study investigates whether the main olfactory and accessory olfactory system are similarly well developed in other marsupials and monotremes at birth ⁄ hatching as in the tammar. Sections of the head of various marsupial and two monotreme species were investigated by light microscopy. Both olfactory systems were less well developed in the kowari and Eastern quoll. No olfactory or vomeronasal or terminal nerves could be observed; the main olfactory bulb (MOB) had only two layers while no accessory olfactory bulb or ganglion terminale were visible. All other investigated marsupials and monotremes showed further developed olfactory systems with olfactory, vomeronasal and terminal nerves, a three-layered MOB, and in the marsupials a prominent ganglion terminale. The main olfactory system was further developed than the accessory olfactory system in all species investigated. The olfactory systems were the least developed in species in which the mother's birth position removed most of the difficulty in reaching the teat, placing the neonate directly in the pouch. In monotremes they were the furthest developed as Bowman glands were found underlying the main olfactory epithelium. This may reflect the need to locate the milk field each time they drink as they cannot permanently attach to it, unlike therian mammals. While it still needs to be determined how an odour signal could be further processed in the brain, this study suggests that marsupials and monotremes possess well enough developed olfactory systems to be able to detect an odour cue from the mammary area at birth ⁄ hatching. It is therefore likely that neonate marsupials and newly hatched monotremes find their way to the milk source using olfactory cues, as has been previously suggested for the marsupial tammar wallaby, rabbits, rats and other eutherians.

Ontogeny and phylogeny of the mammalian chondrocranium: the cupula nasi anterior and associated structures of the anterior head region

“…In newborns, the development of the olfactory system varies in different species [ 70 – 72 ], reflecting differing developmental maturity at birth [ 70 ]. Studies indicate that in some neonate marsupials the olfactory system is capable to detect olfactory cues [ 71 – 73 ]. A well-developed cupula nasi at birth may provide stability in this rostral-most part of the chondrocranium (see above, [ 19 ]).…”

On the development of the chondrocranium and the histological anatomy of the head in perinatal stages of marsupial mammals

“…Freyer [ 94 ] reported nerve bundles in the epithelium rostral to the cupula nasi anterior of Monodelphis domestica , possible nerves of CN V 2 that innervate the area which later develops into the rhinarium [ 108 ]. Previous studies have hypothesized a causal relation between the innervation and sensory anatomy of the area proximal to the cupula nasi anterior and the sensory biology of marsupials at birth [ 109 – 114 ]. Taking eye development as a reference for comparing stages, Elsner [ 115 ] observed that the cupula nasi anterior develops much earlier in Monodelphis domestica than in the tree shrew Tupaia belangeri .…”

Conserved and divergent mechanisms of cortical development across mammalian brain evolution

“…Moreover, the data shown in this chapter revealed that in both dunnarts and mice neuronal production in the piriform context begins, peaks and declines earlier than the neocortex (Figure 3.2C-D). This result supports the hypothesis that the development of olfactory brain areas precedes and might influence the formation of the neocortex across mammals, and further adds to anatomical and behavioural evidence indicating that the olfactory system could be functional during the first postnatal hours, and help marsupial joeys to locate the nipples inside the pouch , Schneider 2011. The temporal definition of the generation of deeper and upper layer neurons with EdU analysis was essential to study the pattern of axonal elongation and the timeline of circuits formation, as it defined the specific stages in which it is possible to transfect and label long-range projection neurons located in L2/3 and L5 with in pouch electroporation .…”